1. Field of the Invention
The present invention is directed to a CoFe soft magnetic thin film alloy having magnetic properties well suited for use in the fabrication of thin film magnetic recording heads and a process for making same. More specifically, the present invention is directed to a CoFe alloy thin film in which the iron constituent is present in a concentration of between about 55 wt. % and 75 wt. % iron and a process for making same.
The application of this high saturation magnetization. CoFe thin film also includes magnetic shields for magnetoresistive read heads, magnetic actuators which may require high moment low Hc materials, application in Micro-Electro Mechanical Systems (MEMS), soft magnetic underlay in magnetic perpendicular recording disk and other applications in electronics. All the above areas require thin electroplated magnetic films because cast materials cannot be used.
2. Background of the Prior Art
The ever expanding demand for greater storage density in magnetic recording systems has correspondingly led to increased demand for magnetic storage media with increased coercivity and higher a real storage density. In order to write to these higher coercivity media it is necessary to have write heads which will put out a higher magnetic flux. Write heads which put out a higher magnetic flux must be made of magnetic pole materials with a higher saturation magnetization.
At present, the highest moment alloy films commercially available are nickel-iron alloys having a saturation magnetization of about 1.75 Tesla. Higher moment alloys are known in the art. These higher moment alloy films are electrodeposited films of an alloy of cobalt-nickel-iron and cobalt-iron-copper having a saturation magnetic moment of 1.9 to 2.0 Tesla.
In spite of these advances in the development of write head-materials, there is a continuing need for the development of magnetic thin films having even higher magnetic moment values.
Although cobalt-iron (CoFe) alloys in the 55-75 wt. % Fe range are known to provide high saturation magnetization, such alloys have been formulated by casting. Obviously, the formation of a very thin film, suitable for use in write head applications, from a cast alloy is not obtainable. The fabrication of such a film by sputtering, vapor deposition and electroplating have been attempted. Sputtering does not produce a film having anisotropic characteristics, which is essential in write head applications vapor deposition results in course grained films which are also unsuitable in this application. Electrodeposition, however, represents a most promising method to form improved thin film CoFe alloy thin films for application as write heads.
Past attempts to produce high-iron CoFe alloy electrodeposited films have failed to generate films having satisfactory magnetic and mechanical properties. Typical electrodeposited CoFe films have high magnetic coercivity, poor or non-existent magnetic anisotropy, saturation magnetization (4 nMs) substantially lower than the theoretical 2.40-2.45 Tesla, and are so stressed that they cannot be plated to a useful thickness (which in write heads is in the 1-3 micron range) without delamination. This poor performance can be ascribed at least in part to the incorporation of oxygen into the electroplated CoFe film during deposition, a characteristic which does not occur when a CoFe alloy is cast. Thus, new thin film CoFe alloys and new processes for making such films which are substantially free of oxygen represents a well established need in the art.
CoFe-containing alloys, including films of such alloys, as well as processes for their manufacture, are known in the art. Examples of the most pertinent of such art are included hereinbelow.
U.S. Pat. No. 4,756,816 describes a process for the electrodeposition of a cobalt-iron alloy on a substrate. This process occurs in an aqueous electrolytic plating bath. The cobalt-iron alloy has an approximate cobalt to iron weight ratio of 90:10. The plating solution also has sodium saccharin, dodecyl sodium sulfate and wetting and buffering agents. The CoFe thin film that results from using the disclosed process has near zero magnetostriction, acceptable permeability for use as a magnetic head, a highly stabilized magnetic domain and approximately 2.0 Tesla saturation magnetization. Magnetic heads fabricated from such films are well suited for use with high coercivity media for high density magnetic recording.
U.S. Pat. No. 5,372,698 is directed to a process of forming a thin film magnetic read/write head having a thin film magnetic core and a substrate upon which a crystalline magnetic alloy of cobalt, iron and boron is electrodeposited. The cobalt-iron-boron core is 90% cobalt and has a saturation magnetization of 1.9 Tesla.
Japanese Patent Publication 6036929 sets forth a wet plating method for making a magnetic thin film. The method involves plating a cobalt-iron-tin alloy in which the tin constituent is present in a concentration of 8 to 25 wt. %.
Japanese Patent Publication 4-229607 discloses a magnetic thin film formed in an electroplating method in which the coating alloy is mainly cobalt but also includes iron in a concentration of 2 to 15 wt. % and sulfur in a concentration of 500 to 2,000 ppm. The sulfur constituent is provided by a sulfinic acid group or its salt, a sulfonic acid group or its salt, a sulfonimide group, a sulfonamide or a thiourea-based compound and mercaptodicarboxylic acid. However, the alloy composition disclosed in this Japanese patent refers to a high cobalt content alloy (85 wt. % to 98 wt %) and does not cover alloys containing lower Co concentration. In addition, although we have no indication of the value of the saturation magnetization, we can estimate its value being below 2.5 Telsa based on the general trend among high cobalt-content CoFe alloys.
Japanese Patent Publication 2-107,737 teaches an amorphous soft magnetic film which includes cobalt, iron, tin and phosphorus.
U.S. Pat. No. 2,654,703 describes the electrodeposition of bright nickel, cobalt, and alloys thereof from acid baths, using aryl (aromatic) sulfinic acids or salts thereof. The plating of cobalt-iron alloys is not claimed; in fact, sulfinates are said to retard the rate of codeposition of iron if ferrous salts are added to such baths, which, if true, would obviously be undesirable in the plating of high-iron cobalt alloys. No mention is made of any magnetic properties or uses of these alloys.
U.S. Pat. No. 5,587,026 concerns a ferromagnetic film which has high saturation magnetic flux density. The film is an alloy of cobalt, iron and a metal selected from the group consisting of tantalum, titanium, zirconium, hafnium, molybdenum and tungsten. The addition of a ternary element to the CoFe alloy system can achieve relatively high saturation magnetic flux density (4 nMs) between −1.5 to 1.9 Tesla and after high temperature annealing a coercivity lower than 1.88 Oersteds. The cobalt content of the ferromagnetic film is 73 at % to 94 at %.
U.S. Pat. No. 4,306,908 is directed to a ferromagnetic amorphous alloy having the formula (CoxNiyFez)aMbGc, where M is Cr, Mo and/or W; G is Zr, Hf and/or Ti; x=1−y−z; 0≦y≦0.2; 0≦z≦0.7; a=1−b−c; 0≦b≦0.5; and 0.05≦c≦0.2.
U.S. Pat. No. 4,933,026 involves a soft magnetic cobalt-iron alloy which includes 0.15% to 0.5% Ta, Nb or mixtures thereof. The cobalt constituent of the alloy is present in a concentration of between about 33% and 55%. This alloy is stated to be operable at temperatures as high as between 960° and 980° C., up to 100° C. above the highest possible operable temperature of earlier approximately equal weight cobalt-iron alloys. The cast alloy of this disclosure is fabricated by vacuum melting and hot rolling a cast ingot to 2.5 mm thickness, reheating the strip to above the order-disorder temperature, i.e. about 800° C., and quenching in a brine solution below 0° C. However, the use of the very high temperatures (above 150° C.) required in the production of CoFe cast alloy is not compatible with the fabrication of magnetic recording thin film heads which makes this CoFe alloy unusable for such an application more generally, the casting process of this patent is not suitable for the preparation of thin film structures.
U.S. Pat. No. 4,925,502 provides a soft cobalt-iron alloy which consists essentially of 35% to 60% by weight Co; 0.03% to 2% by weight Al and the remainder iron. This alloy is formed by compacting and sintering cobalt, iron and aluminum or iron-aluminum powders. The sintering process used in the '502 patent implies the application of very high temperatures (−1400 C.) and is consequently not compatible with the fabrication of magnetic recording thin film heads and the like.
An article by L. H. Chen et al. entitled “Soft-Magnetic Properties of Fe—Co—B Thin Films for Ultra-High-Frequency Applications” in J. Appl. Physics, Vol. 87, Number 9, pp 5858-5860 (May 2000) describes an amorphous CoFeB thin film sputter deposited at a composition of 67 at % iron, 18 at % cobalt and 15 at % boron. Although this thin film exhibits a very low coercivity of about 0.3 Oersted, the saturation magnetization is very low (4 nMs −1.75 Tesla). Moreover, by conventional RF magnetron sputtering technique, a perpendicular magnetic anisotropy tends to occur easily, which would be strongly detrimental to the performance of magnetic recording thin film heads.
An article by S. S. Abd El Rehim et al., “Electroplating of CoFe Alloys from Aqueous Acetate Bath” in Trans. IMF, 2000, 78 (1), pp 41-42 shows CoFe thin films with a composition ranging from about 43% up to about 66% iron (no indication if they are weight or atomic percentages). The alloy composition disclosed in this article has microcracks and is consequently not suitable for the fabrication of magnetic recording thin film heads and the like. Moreover, the authors do not give any indication concerning the magnetic properties of these CoFe alloys.
The full range of CoFe compositions has been investigated by E. M. Kakuno et al. in “Structure, Composition, and Morphology of Electrodeposited CoxFe1-x Alloys” in J. Electrochem. Soc., Vol. 144, No. 9, September 1997, pp. 3222-3226 from an additive-free sulfate-based plating bath. The CoxFe1-x thin films with an alloy composition varying from x=1 to 0 show heterogeneous morphology and composition and are then not suitable for the fabrication of magnetic recording thin film heads and the like. No indication of the magnetic properties of these alloys is given in this article.
In U.S. Pat. No. 4,208,254, the patentees disclose a method of electrodepositing a CoFe alloy having a composition of 7.5-55 wt. % iron the remaining being cobalt, from a fluoride-containing bath. No saturation magnetization value is given in this patent disclosure.
Soft magnetic amorphous CoFeP alloys have been reported in the literature. For example, J. Herretos et al., “Preparation of Fe—Co—P Amorphous Alloys by Electrodeposition” in J. of Non-Crystalline Solids, 201 (1996) pp. 102-109 have electrodeposited a Fe56Co30P14 alloy with a coercivity of about 2 to 5 Oersteds. There is no indication of saturation magnetization. Although amorphous CoFeP alloys have been shown to exhibit excellent soft magnetic properties, they generally have a lower saturation magnetization due to the addition of glass forming agent in relatively high concentrations.
Another type of amorphous CoFe alloy is obtained by the addition of boron as described in U.S. Pat. No. 5,372,698 where the patentees disclose an electrodeposited CoFeB with an iron content between about 7 wt. % and about 12 wt. %, a boron content between about 0.1 wt. % and about 2.0 wt. %, the remaining being cobalt and having a saturation magnetization (4 πNs) of about 1.9 Tesla and a coercivity of about 1.0 Oersted. Despite good soft magnetic properties, the cobalt content (about 90 wt. %) of this alloy is between 25 wt. % and 45 wt. %. Moreover, the alloy of this patent has a low magnetization.
An electrodeposited CoFe alloy containing vanadium is presented by Arcos et al., “Direct and Pulse Current Electrodeposition of Ternary CoFeX, CONiX and NiFeX Thin Film Alloys” in Proc. of the 4th Int. Symp. on Mag. Mat. Proc. and Dev., L. T. Romankiw and D. A. Herman, Jr. Eds, 95-18, (1995), pp. 563-569. CoFeV electrodeposites having a magnetic moment of about 2.0 Tesla are presented in this study. However, no clear indication of the alloy composition and no indication of the coercivity is provided. Furthermore, the saturation magnetization-of about 2 Tesla is too low for this material to be used in the fabrication of advanced magnetic recording thin film heads.
The addition of copper to the electrodeposited CoFe alloy is disclosed by Andricacos et al. in U.S. Pat. No. 5,582,927 where a CoxFeyCuz alloy with a composition in the range of about 65 wt. % to 92 wt. % cobalt, about 6 wt. % to 15 wt % iron and about 2 wt % to 20 wt. % Cu has been electrodeposited. This alloy has a high saturation magnetization (4 πMs) in the range of 1.6 Tesla to 2.4 Tesla and a coercivity of less than 1 Oersted. However, these excellent soft magnetic properties require a very low iron content CoFe alloy, i.e., less than 15 wt %.
In U.S. Pat. Nos. 4,014,759 and 4,053,373 an α-hydroxy sulfone compound, which is the reaction product of an aromatic sulfinate and an aldehyde or dialdehyde, is used in iron-containing baths to improve plating of 15 wt. % to 70 wt. % Fe-containing alloys of nickel or cobalt or nickel and cobalt.
Finally, Liu, et al., Journal of Applied Physics, Vol. 87, no. 9, 5410-5412 (2000) describes the electrodeposition of cobalt-iron-nickel alloy thin films for magnetic recording heads. This deposition occurs in the absence of sulfur-containing additives. Although the coercivity of these films was about 1 Oe, the saturation magnetization was only 2.0 Tesla.